Automotive ventilation and air conditioning systems have historically used air duct opening and closing valves that were swinging doors or flaps. Besides the volume inevitably occupied by such a valve just to accommodate its swinging motion, they present problems in their lack of ability to gradually apportion air flow into or through an opening. By their very nature, such valves tend to be completely open or closed, rather than progressively moving between open and closed, in a predictable fashion. This is generally referred to as lack of linearity.
A newer, as yet not widely used, valve design uses a moving film belt that is shifted back and forth by winding it back and forth between a pair of parallel rollers. As the belt moves in one direction, it winds up on one roller and concurrently winds off of the other roller, and vice versa. An opening in the belt progressively covers and uncovers matching openings within the air flow housing, so as to give a predictable fractional flow, in addition to full on and full off. A thin, shifting belt is also potentially more compact than a series of swinging flaps. An inherent problem with a belt is that it is most convenient to turn both rollers with a single motor, which would generally turn both rollers at the same rate of rotation. However, except for an instant at the midpoint, one roller, either the roller that is winding up, or the roller that is winding off, will have a thicker layer of the belt wound around it than the opposite roller has. The instantaneous rate at which the belt is winding onto a rotating roller depends on the effective radius of the roller at the point where the belt is entering or the wound layer. This, in turn, depends on the thickness of the wound belt layer on the roller at that point in time, and is always changing. Maintaining a constant tension force in the belt depends, in turn, on the winding off roller matching its rate of belt wind out to the other roller's rate of wind up. Unless some means is provided to allow the rollers to match their wind up and wind off rates relative to one another, the belt will inevitably alternately slacken and tighten as it shifts back and forth between the two rollers. It would be possible, of course, to provide a complex control system, similar to tape recording machines, in which each roller can be independently driven when it serves as the wind up roller, and allowed to pull the other roller passively along with the tape, while providing an independently engageable drag mechanism on the wind off roller. However, a simple, single drive motor system would be much more desirable and economical.
This problem has been recognized, and at least one single motor drive mechanism to allow a tension compensating differential rate of rotation between the wind up and wind off rollers has been disclosed in U.S. Pat. No. 5,243,830. There, each roller is paired with a co axial, co rotating conical pulley located at one end, which is oppositely oriented relative to the conical pulley on the other roller. One roller and pulley pair is directly motor driven, while the other is indirectly driven by the first, either through the film or the wire. In one direction, the film is in tension and serves as the opposite roller driving means, while the wire is slack, and vice versa. Within each rollerpulley pair, as the belt winds up more thickly, the wire moves to a smaller pulley radius, and vice versa. Since the conical pulleys are oppositely directed, as the effective radius of one is growing, the other is contracting, and vice versa. The belt winds in the opposite direction of the wire, so that the winding up roller with its thickening wound layer of belt can rotate more slowly as the winding off roller with its thinning wound layer of belt compensates by rotating more quickly. While the mechanism does work with only one motor, it is somewhat complex, with its extra wires and pulleys. Moreover, the mechanism is very sensitive to misadjustment. Unless the counter rotating rollers and pulleys are timed and registered to one another precisely, the wire will not finish winding up simultaneously with the belt winding off, and vice versa. Repair and adjustment within the small, confined spaces available is very difficult. As a consequence, in actual production, the opposed tapers on the pulleys were eliminated in favor of non tapered pulleys. The non tapered pulleys were still used in order to allow one motor to run both rollers, but a conventional window shade type torsion spring was used to provide for wind up--wind off rate matching between the rollers.